This invention relates to an ice cutting system and more particularly to a system for forming a path of unconsolidated ice in an ice floe or ice sheet in order to permit the ice to move relative to a marine structure without applying destructive forces to the structure.
The invention disclosed herein deals with the problem of moving ice encountered in frigid waters such as in the Arctic Ocean. Currently there is considerable activity in these areas directed toward the location and development of sources of petroleum and other natural resources. In the search for petroleum in offshore areas, platforms are typically used to locate equipment and personnel. These platforms are normally maintained in a relatively fixed position with respect to the underwater floor such as by anchoring the platform to the ocean floor or by use of dynamic positioning techniques. In any event, in the normal course of drilling or producing from such a platform into the earth's subsurface, pipes are extended from the platform into the earth's subsurface and it is important to maintain the platform in a relatively fixed position in order to prevent breaking or withdrawing the pipe from the earth's subsurface.
Such platforms, if located in ice covered areas of water, are exposed to ice floes which sometimes float freely on the water and may frequently be of such size that a platform is susceptible to damage or destruction as a result of forces produced by the moving ice. For example, the Arctic Ocean adjacent the North coast of Alaska is characterized by its shallow depth and gradual slope to deep water. Air temperatures usually range from -40.degree. F to +50.degree. F. The water is fairly uniform in temperature, from +28.degree. F to +30.degree. F and very saline. Winds are predominantly from the East, 10 to 15 knots with gusts of 50 to 60 knots. In the months of November through April, large masses of ice, known as ice packs, are in continuous movement under the effects of wind. The huge ice fields are propelled in all directions by the winds and somewhat, although not greatly, by ocean currents.
The main ice formation in the Artic Ocean is an ice sheet which is generally 6 to 10 feet thick. Another form of ice encountered is "rafted" ice which is the term used to describe the overlapping of ice sheets as one sheet rides up over another sheet, resulting in an ice floe made up of two or more distinct layers. However, rafting does not generally take place between sheets of more than 1 or 2 feet in thickness since ice is weak in tension and cannot withstand the deflection necessary for thicker sheets to ride over the other. A more serious hazard is represented by ice ridges which are formed by the motion of ice sheets, and, which can attain, heights in excess of 50 feet. In this regard Arctic ice normally exhibits a compressive strength of 1000-3000 PSI and a tensile strength of 300-1000 PSI depending on various factors. For example, colder and less saline conditions would cause the foregoing strength figures to move toward the higher end of the ranges.
Due to problems inherent to petroleum exploration and production in ice covered regions, considerable effort has been expended toward developing subsea or other alternate systems of operating in such areas. As a result of the high cost of alternative systems coupled with the amount of technical development involved, none to date have become operable. If techniques can be developed to cut ice or otherwise render an unconsolidated path through an ice floe as it moves relative to a more conventional platform, then such conventional offshore systems can be used which are presently available and least expensive in cost.
The amount of ice cutting necessary to prevent damage to a platform from moving ice varies, of course, with changing conditions such as thickness and rate of ice movement. Since the problem of energy supply is extremely critical in arctic operation, minimizing use of energy as for cutting ice is very important. For this reason it would probably be preferable to use a monopod structure to support a platform thereby minimizing the profile subject to ice interference, but multileg platforms may offer other overall advantages and this invention is not limited to any particular type of platform design. In addition, if attention is paid to the size of ice portions being cut or broken from the ice mass, the energy expended in cutting a path through the ice may be minimized. For example, the smaller the particles produced by the cutting process, the higher the energy expended in disaggregating a given volume of it. Furthermore, if ice is shredded or chipped into small particles, it tends to fluff, creating a large volume that, when wetted, tends to freeze, generating an even greater problem. It is preferable to cut the ice into fragments that can be easily moved during ice motion and displaced away from the cutting area such as by fragments piling on top of one another and drifting past a platform or being shoved aside. Therefore, the ideal situation is to cut the largest blocks or fragments that are movable without damage to the structure about which the path is being cut.
In designing a cutting system it is also desirable to minimize maintenance since severe weather conditions make outside maintenance hazardous. A breakdown of such an ice removal system might cause the ice to damage or destroy the platform.